The range of warm-white color temperatures, i.e. below 3500 K, has hitherto been difficult to realize using commercially available LEDs. The standard phosphors produce a color temperature of more than 5000 K. Therefore, it has hitherto been attempted to produce warm-white color temperatures in a very complex way by combining a plurality of LEDs cf., for example, WO 02/52901 and WO 02/52902.
Simple LEDs, aimed at producing warm-white luminous colors have hitherto been based on UV chips. On account of the considerable energy difference between the UV region and the short-wave visible region (blue), and also on account of the UV-induced, which is higher on account of the higher energy, radiation-induced more rapid aging of the housing and of the phosphor potting, these LEDs fail to achieve either the service life or the efficiency of neutral-white LEDs based on blue-emitting chips as have hitherto been available.
One alternative is RGB-LEDs based on luminescence conversion LEDs comprising sulfide and thiogallate phosphors, cf., for example WO 01/24229. However, it has been found that the phosphor proposed in that document does not satisfy the requirements with regard to long-term stability and efficiency when using high-power chips which reach a high operating temperature. The sulfides are chemically unstable with respect to moisture and the thiogallates proposed have a pronounced temperature quenching. Moreover, on contact with water, the known sulfide phosphors decompose to form toxic gases, such as hydrogen sulfide.
An alternative solution which can be demonstrated to have a long service life and a high efficiency combined with a very good color rendering index has not hitherto been disclosed. The use of a mixture of the known YAG:Ce and a red phosphor, such as for example Sr2Si5N8:Eu only leads to maximum Ra values of 85 to 90, cf. in this respect, WO 01/40403.
Phosphors of the oxynitridosilicate type are known per se under the shortened formula MSiON; cf. for example, “On new rare-earth doped M—Si—Al—O—N materials”, J. van Krevel, T U Eindhoven 2000, ISBN 90-386-2711-4, Chapter 6. They are doped with Tb. Emission is achieved under excitation by 365 nm or 254 nm.
A new type of phosphor is known from the as yet unpublished EP patent application 02 021 117.8. It consists of Eu- or Eu,Mn-coactivated oxynitridosilicate of formula MSi2O2N2 (M=Ca, Sr, Ba).